A great wiki on many things released can be found at https://bulbapedia.bulbagarden.net/wiki/Main_Page and I use it often as a reference when I add new sets on here.
A great way to get an idea of cost on cards is tcgplayer.com which is now owned by eBay but a great resource on finding what the cost of English TCG cards are.
As others mentioned it depends if she is more into the trading card game or actually wanting cool cards of their favorite Pokemon. Anyone actually wanting to play the game can get any card for extremely cheap, only the promo, alternate art, etc cards fetch high dollar amounts.
Find me on YouTube - BreakingTheSeal - Breaking the seal on trading cards, video games, and other collectibles. Also Instagram - BreakingTheSeal_YT for other related content.